Pressure is a known value if you do well the tests and multiple mounting are always here to ensure consistency, no matter than TIM is changing. TIM doesn't change radically in 2 days to give a 3°C gap, this is a false pretext. Perhap's change will be visible in 2 years because you are doing only 1 or 2 tests a month and what will be the variation at these 2 different moments? 0.01°C? 0.1°C? 1°C? more? Nobody has the answer because nobody made measurement (don't think Bill made such test?) and it's always a special case anyway, no global statement. But to ensure no matter, you make a flash solder on a ultra thin thickness between IHS and die and no problem anymore.
IMO, it's the responsability of the tester to program a maintenance period of his die : you fully test a WB and one year later, you test it again and compare, how results will be? How much variations I get? Is die worn? Is TIM changed? etc. etc. A lot of interesting stuff here.
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